Clipping system



Nov. 26V, 1946. J. A. RADQ GLIPPING SYSTEM 511ml May 14, 1943' Patented Nov. 26,1946' level.

UNITED STATES Pirrizur omer? sohn s. man, Greatness, N.

asdgnments, to `Hase Y., assigner, by ltine Research.

Inc., Chicago, lll.. a corporation of Illinois Application May 14, 1943. Serial No. 487,007

(Cl. o-27) 6 Claims. l Y This invention relates to a system for asymmetrically clipping an applied signal, and particularly concerns an arrangement for eilectively clipping all of the lvideo-frequencycomponents from a composite television signal which includes synchronizing-signal components and associated video-frequency components.

For some purposes it is desirable to employ a vacuum-tube clipper which is adaptedto translate the desired portion of a wave signal and to clip the signal sharply at a predetermined signal Clippers of the anode currentv cutoff type are generally most eiiective for this purpose. In, certain television systems which employ clippers of the general type under consideration. it has been found that more than one clipping stage -is necessary toclip completelythe unwanted portions of the wave signal. These clipping arrangements ordinarily'utilize an intermediate stage for amplifying a wave which was previously clipped at a predetermined level by the first clipping stage. 'I 'his intermediate stage also reverses the polarity of the output wave of the first clipping stage. whereby the vamplified wave is presented with the proper Apolarity for clipping in the second stage. This last-mentioned stage clips the amplified wave and usually separates completely the video-frequency components from the synchronizing-signal components. In some multi-tube arrangements, clipping is accomplished by the last tube only, the preceding tubes serv- It is another object of the present invention to provide an improved multi-tube clipper of the anode current cutoil' type which eliminates the need of an intermediate amplifier stage but which that the aforesaid peaks are effective to increase` .the conductance of the tube. An anode load impedance is coupled to the anode of the first tube. 'I'he system also includes means for causing the peaks of the applied signal to be stabilized in the input circuit of the first vacuum tube, the amplitude of the applied signal being so related to the operating characteristics 'of the first tube that anode-current clipping is effected therein, a second vacuum tube having input and output electrodes, and means effectively comprising only passive coupling elements coupling the above-mentioned load impedance to the insecond tube. Furthermore. the system includes means for causing the recurrentpeaks ofxthe signal to be stabilized in the input circuit ofthe second tube. The amplitude of this signal which l is applied to the second tube is so relatedjto the operating characteristic of the second tube that anode current clipping is effected in the second tube, The system also includes a means coupled to theoutput electrodes of the second tube for deriving'ra clipped output s'i'g'nal.

. For a better understanding of -the presentinvention, together with other and further objects thereof, reference is had to the following descripj tion taken in connection with the accompanying drawing, and its scope will be pointed out inl the appended claims.

In the accompanying drawing, Fig. 1' is acir-' Y cuit diagram, partly schematic,` of a complete' television receiver including a vacuum-tube clipping system embodying the inventionywhile Fig. 2 is a graph of a tube characteristic used in explaining the operating characteristics of the clipping system of Fig. l.

Referring now more particularly to Fig. 1, thev I system illustrated comprises a television-signal includes the advantages of an ampliiier stage.

In accordance with the present invention, there is-provided a systemfor asymmetricallyl clipping an applied'signal having recurrent peaks comprising a first vacuum tube having input and output electrodes, and means for applying the signal to the input electrodes with such polarity receiver of the superheterodyne type including an antenna system I 0, II connected to a radiofrequency amplifier I2 of one or more stages. I to which is connected in. cascade, in the order named..an oscillator-modulator I3, an intermediate-frequency amplifier I4 of one or more stages, a detector and A. V.YC. source I5. avid'eo'f frequency ampliiier IB of one or more sta andf Aanimage-reproducing device I1. A linVV scanning circuit I8 and a iield-scanning'circult I9 are coupled to the output circuit of ther videofrequency amplifier I6 through a clippingsys- 4 tem- 22 and an intersynchronizing-signal sepannees arator 20. The output circuits of the line-scanning circuit I8 and the field-scanning circuit It, respectively, are coupled to the line-scanning windings 23, 24 and the held-scanning windings 25, 26 of the image-reproducing device Il. The image-reproducing device i1 includes a conventional electron-gun structure 21 and afluorescent screen 28 and is provided with a suitable source of unidirectional voltage 29. An automatic amplifcation-control potential derived from unit I is applied to one'or more tubes of radio-frequency amplier I2, oscillator-modulator I3, and intermediate-frequency amplifier I5 by way of the connection indicated as A. V. C. in a conventional manner. A sound-signal reproducer 2l of conventional design is coupled to an output circuit of intermediate-frequency amplifier It. The stages or units Ill to 2I, inclusive', may all be iof conventional construction so that detailed 'illustration and description thereof are deemed unnecessary herein.

Referring briedy to the operation of the system described above, television signals intercepted by antenna circuits Iii, II are selectedv and amplified in radio-frequency amplifier I2 and coupled to the oscillator-modulator I3, wherein they are converted to intermediate-frequency signals which, in turn, are selectively amplied in the intermediate-frequency amplifier I4 and delivered to the detector I5. The modulation components of the signal are derived by detector I5 and are supplied to the video-frequency ampliiler I6 wherein they are amplied and fromwhich they are supplied, in the usual manner, to a brilliancy-control electrode of the image-reproducing device I1. Unstabilized modulation components of the signal are also supplied by the video-frequency ampliiier I6 to the clipping system 22 which is connected in circuit with the intersynchronizing-signal separator wherein synchronizing signals ofdiierent types are separated and applied to the synchronizing` control elements of circuits It and It. The intensity of the scanning ray of device I1 is modulated or controlled in accordance with the video-frequency voltages which are applied to the control grid in the usual` manner. Scanning waves are developed in line-scanning circuit I and eld-scanning circuit'IS and are applied to the scanning elements of lthe image-reproducing device I1 to produce electric vscanning fields, thereby to deflect the scanning -ray in two directions normal to each other so as to trace a rectilinear pattern on the screen, thererby to reconstruct the transmitted image. Sound signals accompanying the television signals are reproduced by unitl 2| in a conventional manner, while the bias derived from unit I5 and applied to the precedingA stages serves to maintain the signal-input amplitude todetector I5 within a relatively narrow range for a wide rangeof received-signal intensities.

Referring now to the portion oi' Fig. 1 embodying the present invention, there is provided a system 22 for asymmetrically clipping an ap plied signal having recurrent peaks. This system comprises a first vacuum tube such as a triode 'having input and output electrodes, and means for supplying a signal output from amplirler I6 to the input electrodes of the tube with such polarity that the peaks of the applied synchronizing signal are effective to increase the conductance of the tube.- This means comprises a coupling condenser 32 connected between the control electrode of tube 30 and an input terminal 3l and a grid-leak resistor 33 in circuit a triode and which preferably has a sharp anode current cuto characteristic, has its cathode connected directly to the anode of tube 30 while its control electrode is connected to ground through a condenser 31. A grid-leak resistor 36 is connected between the control electrode and cathode of tube 35. The time constant of resistor 3S and condenser 3l is'long compared to the synchro nizing-signal period to provide a self-biasing or stabilizing circuit for tube 35. The anode of the second vacuum tube 3E is connected to the :iource +B through a load resistor 39 and to an output terminal d@ through a coupling condenser 4I.

The system 22, therefore, includes means ,effectively comprising only passive coupling elements which couple the load impedance 38 to the input electrodes of` the second tube 35, for applying to the second tube the signal translated by the first tube with' such polarity that the recurrent peaks of the synchronizing signal are effective to increase the conductance of the second tube. By passive coupling elements are meant those elements which include no power amplifying devices. As just described, the passive coupling means vbetween the tube '30 and the tube 35 comprises the connection between the anode of tube 30 and th'e cathode of tube 35, and the grid-leak condenser combination 36, 31 which is connectedto the control electrode of the latter tube.

In considering the operation of the portion of Fig. 1 comprising the present invention, it will first be assumed thatl an unstabilized television signal, as shown at the upper left of tube 30, is applied to the input circuit thereof from the video-frequency amplifier I6. In this composite signal, recurrent positive peaks or tips 42 represent the synchronizing-signal components and the lower irregular portions t3 represent video-frequency components which are to be removed by the 'clipping system before the resultant signal is translated to the intersynchronizing-signal separator 20. Grid rectification takes place in the input circuit of tube 30 and develops a grid bias which stabilizes the peaks 42 of the input signal substantially at the zero-voltage level of the grid voltage-platecurrent characteristic of the tube.

The recurring positive peaks of the composite input signal arel effective to increase the conductance and the space current of the tube 30. The negative peaks of the translated signal developed across load resistor 38 are applied to the input circuit of tube 35 with the'peaks effectively positive at the grid due to theA fact that the grid of this tube is effectively grounded with respect to the synchronizing-signal component while its cathode potential iluctuates in accordance with the signal applied thereto. They time constant of themresls'tor-condenser combination 36, 31 is long with respect to the synchronizing-signal period so that the signal is effectively stabilized at the zero-voltage level of the grid voltage-plate current characteristic of tube 35 as shown in Fig. 2. The amplitude of the signal applied to the second tube 35 is so related to its grid voltage-plate current characteristic that anode current clipping is effective in the second tube to produce an output signal in which video-frequency components are entirely absent. The first tube 30 also preferably has a sharp cutoii characteristic and partial clipping by anode current cutoff is preferably effected therein although the tube may operate primarily as an amplifier.

'I'he load resistors 38 and 39 are preferably so proportioned that the resistance value of resistor 33 is a small fraction of the resistance value of resistor 38. Also the circuit is preferably so proportioned that the sum of the resistance values of resistor 39 and the minimum anode-cathode resistance of tube 35 is. small with respect to the value of resistor 38.

Due to this proportioning of load limpedances, a. substantial variation of Athe gain of the first tube 30 is provided by the action of the second tube 35 which enhances the effectiveness of the clipping action accomplished by the system. The space current path of tube 30 during the intervals -of the synchronizing pulses comprises resistor 38 shuntedby the anode-cathode resistance of tube synchronizing signals are stabilized at the zero grid-voltage level of tube 35., the video-frequency portions of the signal are shifted Wellbeyondv the cutoff point of tube 35, resulting ina clean synchronizing-signal output.

This action is illustrated in Fig. 2 which shows a. portion of a television signal 52 which is related to the grid voltage-plate current characteristic of the second tube to illustrate the action if the above-mentioned optimum proportioning of load -impedances is not used. A portion of a second television signal 50, shown in dotted lines, is also illustrated to indicate the action where the optimum proportioning is provided. It will be seen that signal 52 includes residual video-frequency components which are not removed by anode current clipping in the second tube, while the entire undesired video-frequency components of signal 50 are displaced well beyond the cutoff level so that an ideal synchronizing signal is translated by tube 35 to its output circuit.A

The control electrode of tube 35 also effectively provides a means for reducing capacitive coupling between the cathode and anode electrodes of that tube thereby to prevent coupling of `videofrequency components, developed across load resistor 38, to output terminal during the times tube-35 is nonconductive. This obviates the need of a multi-grid tube, for example, a screen-grid tube which is required in many clipping systems. It also assures a clean base line in the clipped synchronizing signal, and eliminates misiiring of associated synchronizing-signal apparatus by random `high-frequency signals which might otherwise be coupled through by capacitive coupling at this point. I

In the event a stabilized television signal is applied to the input circuit of the system, such as 'from the detector and A. V. C. source I5, a

be of suflicient magnitude to displace the videofrequency components of the applied signal beyond the cutoff level in tube 30, as is well understood' by those skilled in the art.

It will also be understood that the system is operative even though unstabilized signals are applied to the first tube 30 and the first tube is operated with a fixed bias on its control electrode. Under these conditions the first tube acts as an amplifier for the unstabilized signal and translates it into the second tube 35 where it is stabilized and clipped substantially as described.

. While applicant does not Wish to be limited to any particular circuit values for the embodiment of the invention described,` the following set of representative values may be utilized in the circuit comprising the instant invention: Tubes 30 and 35 'Iype 12SL7GT double triode Voltage supply +B volts-- 250 Condensers 32. and 31, each microfarad- 0.1 Condenser 4| do 0.01 Resistors 33, 36 and 38, eachy megohms 2 Resistor 39-, ohms-- 50,000

While the invention has been illustrated and may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A system for asymmetrically clipping an applied signal having recurrentpeaks comprising, a first vacuum tube having input electrodes and output electrodes including an anode, means for Aapplying said signal to said input electrodes ywith such polarity vthat said peaks are effective to increase the conductance of said tube, an anode load impedance coupled to said anode, means for causing said peaks of said applied signal to be stabilized in the input circuit of said first vacuum tube, the amplitude of said applied signal being so related to the operating characteristics of said first tube that anode current clipping is effected therein, a second vacuum tube having input and output electrodes, means effectively com- -prising onlyI passive coupling elements coupling said load impedance to said input electrodes of said second tube for applying thereto a signal translated by said fir-st tube` with such polarity that said peaks are effective to increase the con'- ductance of said second tube, means for causing said peaks to be stabilized in said input circuit of fixed bias from a suitable source may be substi- 'Y tuted for the stabilizing means comprising the condenser grid-leak arrangement 32, 33. The grid bias provided by this source would preferably said second tube, the amplitude of the signal applied to said second tube being so related toits characteristic thatanode current clipping is effected in said second tube, and means coupled to said output electrodes of said second tube for deriving a clipped output signal.

2. A system for asymmetrcally clipping an applied signal having recurrent peaks comprising, a rst vacuum tube having input electrodes and output electrodes including an anode, means for applying said signal to said input electrodes with such polarity that said peaks are effective to in- 7 crease the conductance of'said tube, an anode load impedance coupled to said anode, a second vacuum tube having input and output electrodes including an anode, cathode, and a control electrode, means including a conductive connection between s'aid anode of said irst tube and said cathode and eectively comprising only passive' trode of said second tube for signals of the frequency of said recurrent peak portions of said applied signal, the amplitude of the signal applied to said second tube being so related to its characteristic that anode current clipping is effected in said second tube, and means coupled to said output electrodes of said second tube for deriving l a clipped output signal.-

3. A system for asymmetrically clipping an applied signal having recurrent peaks comprising, a ilrst vacuum tube having input electrodes and output electrodes including an anode, means for applying said signal to said input electrodes with such polarity that said peaks are effective to increase the conductance of said tube, an anode load impedance coupled to said anode, means for causing said peaks of said applied signal to be stabilized in the input circuit of said nrst vacuum'tube, the amplitude of said applied signal being so related to the operating characteristics of said iirst tube that anode current clipping isr effected therein, a second ,vacuum tube having input electrodes and output electrodes comprising a cathode and an anode, a load impedance for said second tube, said last-named impedance and the anode-cathode path of said second tube being effectively connected in series across said firstnamed impedance, means effectively comprising only passive coupling elements coupling said output electrodes of said first tube to said input electrodes of said second tube for applying thereto a signal translated by said ilrst tube with such polarity vthat said peaks are eil'ective to increase the conductance of said second tube, means for causing said peaks to be stabilized in said input circuit of said second tube, the amplitude of the signal applied to said second tube being so related toits characteristic that anode current clipping is effected in said second tube, and means coupled to said output electrodes of said second tube for .deriving a clipped output signal.

4. A system for asymmetrically clipping an ap plied signal having recurrent peaks comprising,-

aiirst vacuum tube having input electrodes and output electrodes including an anode, means for applying said signal to said input electrodes withv such polarity that said peaks are eifective to increase the conductance of said tube, an anode load impedance of predetermined magnitude coupled to said anode, a second vacuum tube havtude which is small with reference to that o! said mst-named impedance, said load impedance of said second tube and said anode-cathode impedance being connected across said mst-named impedance, means effectively comprising only passive coupling elements coupling said output electrodes of said first tube to said input electrodes of said second tube for applying` thereto a signal translated by said ilrst tube with such polarity that said peaks are effective to increase the conductance of said second tube, means for causing said peaks to be stabilized in said input circuit of said second tube, the amplitude of the signal applied to said second tubebeing so related to its characteristic that anode current clipping is eilected in said second tube, and means coupled to said output electrodes of said second tube for deriving a clipped output signal.

5. A system for asymmetrically clipping an applied signal having recurrent peaks comprising,

a first vacuum tube having input and output electrodes, means for applying said signal to said input electrodes with such polarity that said peaks are eilective to increase the conductance oi said tube, a second vacuum tube having an inputelectrode and output electrodes including a cathode and an anode, means comprising said last-mentioned input electrode for reducing capacitive .couplingfbetween said cathode and said anode oi signal components translated by said nrst tube, means .eiectively comprising only passive coupling elements coupling said output electrodes of' said ilrst tube to the input circuit o! said second tube for applying thereto a signal translated by said ilrst tube with such polarity that said peaks are effective to increase the conductance of said second'tube, means for-.causing said peaks f to be stabilized in said input circuit of said second tube, the amplitude of the signal applied to said second tube being so related vto its charac-v teristic that anode current clipping is eilected in said second tube, and means coupled to said output electrodes of said second tube for deriving a clipped output signal.

8.v A system for asymmetrlcally clipping an applied signal having recurrent peaks comprising, a first vacuum tube4 having input and output electrodes, means for applying said signal to said input electrodes with such polarity that said peaks are effective to increase the conductance of said tube. a triode second vacuum tube having an input'electrode and output electrodes including a cathode and anode, means effectively comprising only passive coupling elements coupling said output electrodes o! said first tube to the input circuit o! said second tube for applying thereto a signal translated by said first tube with such polaritythat said peaks are eiiective to increase the conductance of said tube, means for causing said peaks to be stabilized in said input circuit of said second tube and for effectively grounding the control electrode thereof for alternating currents, the amplitude of the signal applied to said second tube being so related to its characteristic that the anode current clipping is effected in said second tube, and means coupled to said output electrodes oi' said second tube for deriving a clipped output signal.

' JOHN A. RADO. 

